The mur is a service that pharmacies offer on behalf of the nhs to review people's medication. Basically to check that they know what, and why they are taking the meds they have prescribed. It is also a chance to check that they need everything. Any drug interactions should be picked up by the pharmacist or dispensers when being dispensed. But the mur is another chance to check.
I work in a pharmacy and have heard of one mur where a patient who has been getting inhalers for years hasn't been using them properly. They had been dispensing a dose into the air then sucking in the cloud... totally useless and therefore the mur helped.
Also it helps with patient waste. We had one patient who moved away and the new owner of their house brought back 5 black bin bags of stuff that we had dispensed. Some of it was over 8 years old. This person also ordered all their own medication and would regularly kick off if we didn't have something in stock which we later found out they had about a years supply of at home. I think the total amount we had to destroy ended up being over £10000
Also the majority of drugs come in 28 packs. There are a few 30's. When you get given the next collection date it will be 28, 56 or 84 days into the future. In the "pharmacy year" there are 13 months. The amount of time we waste daily on explaining that although there isn't enough tablets for a calendar month in your bag, the next prescription will be here and ready before you run out as we work to a 28 day month. It would be a he'll of a lot more work for the already over worked and underpaid doctors, dispensers and pharmacists to constantly be changing between dispensing 28,29,30 or 31 tablets rather than just mostly standardise it to 28.